Weather 15 April 2016

What Happened There are several outstanding features on this week’s synoptic progression. Most important is the persistent high pressure control on the surface covering almost the entire southern Africa except for a small sliver of lower pressure along the northern Namib coastline. The second noteworthy feature is the presence of a tropical depression about 1000 km east of the northern tip of Madagascar. This vortex has been in the making for about ten days, gradually developing in intensity. Earlier this week it became sufficiently strong to be named Tropical Cyclone Fantala with a core pressure of only 972 hPa and 70 knot winds emanating from the core. As the week progressed it gyrated closer to Madagascar and by the Friday, it was only some 600 km away from the Malagasy coastline. Due to the strong high pressure control in the south and over eastern Africa, the zonal flow from the Indian Ocean was blocked. In fact, airflow in the Mozambican Channel tended to be from south to north. This interferes with the normal advection of moisture from the Indian Ocean across central Africa, and eventually into Angola and Namibia, after completing a very wide curve. The result is clear, high-pressure controlled, low humidity conditions over Mozambique, and consequently over Zambia, Zimbabwe and Angola. These countries are all in the moisture pathway from the Indian Ocean to the interior of southern Africa. However, it was noticeable that as Fantala approached Madagascar, the closer proximity to the continent had a marked effect on zonal airflow in the mid and upper levels, from around 15,000 feet to 45,000 feet. By the end of the week, the high pressure control over eastern Africa has been broken and the usual advection pathway restored. This was evident in the “deeper” atmosphere over eastern Africa with considerable vertical extent. Over Namibia, weather conditions continued to be divided in a western half and an eastern half. With high pressure control in the east, all the territory from Angola through Namibia, Botswana and most of South Africa, had early morning barometric pressures around 1016 mB. If one divides Namibia along a north south line from Eenhana to the Fish River Canyon, it more or less indicates the west/east split. West of this line, a low pressure system originated from Angola and slowly crept down the coastline. By Friday this coastal low reaches Lüderitz. The effect is a major pressure differential between east and west (high in the east, low in the west). The result is a pronounced airflow from high to low, i.e. from east to west, leading to fresh winds over the northern Namib, and along the escarpment, all the way south to the Orange River Valley. As these winds descend to the coastal plain, considerable diabatic compression takes place, releasing massive amounts of energy, and Walvis Bay had it first portend of Oosweer on Thursday, continuing into Friday. Overall, the airflow remains north-east, and the low pressure system along the coast continues to grow in extent (southward) and intensity (down to 1012 mB) What’s Coming The weekend brings a continuation of the conditions during the week. The airflow remains from the north-east, and will grow in intensity. This movement is in no small measure amplified by Fantala which drives a strong zonal flow from the Mozambican Channel across Zambia and Angola. It also advects moisture. The coastal low develops into a proper trough, aided considerably by the low pressure system south-west of the continent. The weekend should be windy, the more the further north and north-west. By Sunday evening the trough is well established, running along its customary trajectory from southern Angola through western Namibia into the Northern Cape. Unusual about this trough is that it is located so far west, even going offshore for a considerable distance (some 300km). In the immediate absence of the next South Atlantic high pressure cell, the trough has all the potential to bring in tropical air from the north. It is likely that it will remain in situ for about four days, Monday to Thursday. It will also gradually migrate to the east and the south and may lead to favourable conditions for light rain in the Karas Region and the Kalahari.